1. Field of the Invention
This invention relates to the field of data processing systems. More particularly, this invention relates to the field of the control of access permissions to memory pages within the memory address space of a data processing system.
2. Description of the Prior Art
It is known to provide data processing systems with memory management units responsive to page table data to control the access permissions to pages (continuous regions) of memory. As an example, a particular memory page may be marked as accessible for reads and writes in a privileged exception level, but marked for only read access in a user exception level. Further access permissions which may be specified in addition to read and write access are the ability to execute program instructions read from a memory page.
It is known from the PowerPC processor architecture to provide page table data including access control bits which independently and orthogonally specify for each of a privileged level and a user level whether read access, write access and execute access is permitted to a given memory page. This consumes six access control bits per memory page.
As the amount of memory being used within data processing systems is becoming larger and with the advent of techniques such as virtualisation requiring multiple sets of page table data, the size of page table data has become a more significant factor. Reducing the amount of memory space needed to store the page table data by reducing the number of access control bits used is advantageous. Another factor in this regard is the desire to support different modes of operation using different instruction sets. In order to reduce the amount of memory space consumed by page table data, it is desirable that the page table data used by a newly introduced mode, such as a mode using a newly introduced instruction set, should be backward compatible with preceding page table data and a preceding instruction set.